Fuel system of gas turbine engines



Oct. 23, 1956 J. o. P. HUGHES ET AL 2,767,546

FUEL SYSTEM OF GAS TURBINE ENGINES Filed Sept. 13. 1954 United States Patent 2,767,546 FUEL SYSTEM OF GAS TURBINE ENGINES Application September 13, 1954, Serial No. 455,706

Claims priority, application Great Britain I October 14, 1953 1 Claim. (Cl. 60-39.28)

Leicester, Company England, a British com- This invention relates to the control of the fuel supply to gas turbines having fuel burners of the so-called spill control type in which a supply of fuel enters the side of the burner and is led tangentially into a chamber in which it circulates with relatively high angular velocity prior to discharging through two alternative orifices of which one is the actual burner nozzle and the other leads back to the fuel pump inlet through a control valve.

For example, the invention refers to gas turbines of this type in which the fuel pump is of a positive displacement type and is mechanically driven from the turbine-compressor shaft of the gas turbine. A mechanically-driven pump of this type is convenient in many ways but it suffers from the disadvantage that if the gas turbine-compressor shaft has to operate at variable speeds the pump delivery varies in a manner which dilfers from the requirements of the engine.

In a simple spill-control system such a mechanicallydriven pump might have its outlet connected to the inlet of the burner and arranged so that the output of the pump at idling speed is suflicient to supply the fuel required by the engine and the spill flow required for good atomisation, but at all higher speeds the pump delivery would then exceed the requirements of the engine by a varying amount and the spill would be adjusted to pass these amounts. The smallest size of pump would be that pump which matched the requirements of the engine at full load but such an arrangement would result in instability at some intermediate point where the amount by which the delivery of the pump exceeds the engine requirements is a maximum since at such a point a reduction in spill flow might correspond to either an increase or decrease in engine speed. This instability may be avoided by fitting a pump of such a size that this point where the spill is a maximum occurs at full load but this necessitates a large pump which would absorb much power at full load in supplying the spill flow, and furthermore to obtain this large spill flow the pump would have to work at high pressure.

Alternatively a relief valve might be fitted in the spill line so as to keep the pressure in the spill line at some predetermined value but this value would then control the speed of the engine, and in order to raise this speed, the pressure in the spill line would have to be raised. However, the pump delivery pressure, being dependent upon the engine speed, would increase more slowly which would result in a drop in spill flow and an increase in burner delivery which might easily exceed that required to cause the desired acceleration, and might lead to serious overheating.

Yet an other alternative would be to supply fuel to the burner at constant pressure, but this would entail a very large pump since the spill flow at idling would exceed the delivery flow to the engine at full load.

It is an object of the present invention to avoid these disadvantages, and to allow a mechanically-coupled pump to be used, the delivery of which only slightly exceeds the demand of the engine.

According to the invention we provide a fuel system of a gas turbine operating at varying rotational speed, comprising in combination: a spill control burner, a spill line leading from the said burner to drain, a fuel pump mechanically coupled to the said gas turbine, a fuel supply line connecting the inlet of the said burner to the outlet of the said pump, an automatic pressure reducing valve and a restrictor device responsive to gas turbine inlet temperature arranged in series in the said spill line, the said reducing valve and restrictor device keeping the spill flow through the said line substantially contant, and a control means in the said fuel supply line controlling the pressure at which the fuel is supplied to the said burner.

In order that the invention may be clearly understood and readily carried into effect an embodiment thereof will now be described by way of example with reference to the accompanying drawing in which: 7

Fig. 1 is a simplified diagram of the fuel system of a gas turbine engine according to the invention.

Fig. 2 is a more detailed diagram of this fuel system.

Referring first to Fig. 1, fuel is supplied through a supply pipe 1 to an engine driven pump 2, the discharge pipe 3 of which is connected back to the supply side thereof through a relief valve 4. The discharge pipe 3 is also connected through a manual control valve 6 to the inlet side of the spill burner 7, the spill line 8 of which is connected through an automatic reducing valve' 9, a restriction l0 and pipe 5 to said supply pipe 1.

For example, a gas turbine engine in which the maximum rotational speed is three times the idling speed may have a mechanically coupled fuel pump with a maximum.

capacity of 5 gallons per minute at full speed. Then the capacity of this pump at idling will be approximately 1.67 gallons per minute. Further, if this engine has a fuel requirement of 3.1 gallons per minute at full load and 0.3 gallon per minute at idling, then the spill control might be set to give a constant spill flow of 1.1 gallons per minute. This would result in a flow to the burner of 4.2 gallons per minute at full speed and 1.4 gallons per minute at idling speed both of which are within the capacities of the mechanically coupled pump. The relief valve 4 may be set to open at a pressure of 600 pounds per square inch.

Referring now to Fig. 2, a fuel tank 11 is arranged below the level of the burners 7 and connected through a filter 12 to the supply side of the pump 2 which is mechanically coupled to the engine. A spring loaded relief valve 4 is arranged between the discharge pipe 3 of the pump 2 and the supply pipe 1, and may be set to open at, for example, 600 lb./ sq. in.

The pipe 3 leads to the manual control valve 6 which contains a throttle valve 16 and a shut down valve 26. The throttle valve 16 has a continuous range of throttling positions varying from an idling position A indicated in dotted lines to a full load position B indicated in full lines, while the shut down valve 26 is normally in a closed position C as indicated in full lines but when put into position D shown in dotted line provides a by-pass to the supply side of pump 2 through pipes 15 and 5, and the casing of valve 4.

Between the throttle valve 16 and the shut down valve 26, the said manual control valve 6 is connected toa trip device 17 for lubricating oil failure and then through a loading valve 18 and filters 19 to the burners 7.

The interior of bellows 37 of the trip device 17 is exposed to lubricating oil pressure so as to keep the valve 27 closed as long as the hydraulic force produced by the lubricating oil is greater than the hydraulic force produced by the fuel on the face of the valve 27. In the event of a failure of lubricating oil pressure the valve 27 lifts and the fuel is by-passed through pipes 47 and 25 and 5 to the pump supply.

The loading valve 18 contains a capsule 28 partly filled Patented Oct. 23, 1956- with liquid which liquid safeguards the capsule 28 from damage when the pressure of the fuel oil is high,.say of the order of 600 lb./sq. in. This loading valve 18 is arranged so as to require a pressure of approximately 60 lb./ sq. in. on the valve face to lift the valve but, once it is lifted, the fuel. pressure acts on the capsule 23 and keeps the valve open until thefuel pressure drops to a low value of, say 30 lb./sq., in. When the fuel falls below this minimum value, the, valve 17 positively cuts oil the fuel supply to the burners 7.

The spill lines 8 of the'burners' 7 are connected to an automatic reducing valve 9 arranged to maintain the pressure before the restrictor 10 at some constant value and so to ensure a constant spill flow through the said restrictor 10 as long as the pressure in the spill lines 8 is above the said constant value. The valve 9 is adjustable so that the value at which the spill flow is kept constant may be chosen.

The restrictor 10 is preferably constructed as a modulating valve controlled by the temperature of the engine combustion chamber through lines 110 and 111. This temperature-controlled modulating valve fully opens the spill line behind the reducing valve 9 when the temperature in the combustion chamber. exceeds a predetermined value. When the spill line is thus opened, the spill flow is increased and the flow from the burner to the combustion chamber is correspondingly reduced, thus causing the temperature to fall until it reaches a predetermined value, when the valve 10 again restricts the orifice. In a gas turbine plant having more than one combustion chamber, there is a modulating valve 10 for each chamber arranged in parallel such that whichever chamber exceeds the predetermined value, causes. the spill lines to be opened. Thus, in Fig. 2, line 110 would be connected to one combustion chamber and line 111 to the second chamber of an engine having two combustion chambers;

Instead of joining the spill and using a single automatic reducing valve 9 lines 8 of the two burners and two modulating valves 10 operating in parallel as shown in Fig. 2, there may be separate spill lines, separate reducing valves and separate modulating valves, one for each combustion chamber. The by-pass pipe 25 may, however, be common to both modulating valves.

It will be noted that the fuel control system according to the present invention dispenses with the usual tip shut-offs of spill burners, which shut-oils are necessarily exposed to high temperature and therefore liable to difficulties in maintenance. Furthermore, with constant spill flow the total amount of fuel entering the burner varies with the amount entering the combustion chamber, which simplifies the design of a burner to give good atomisation over a range of burner inlet pressures and fuel flows.

What we claim as our invention and desire to secure by Letters Patent is:

A fuel system of a gas turbine operating at varying rotational speed, comprising in combination: a spill con trol burner, a spill line leading from the said burner to drain, a fuel pump mechanically coupled to, the said gas turbine, a fuel supply line connecting the inlet of the said burner to the outlet of the said pump, an automatic pressure reducing valve and a restrictor device responsive to gas turbine inlet temperature arranged in series in the said spill line, the said reducing valve and ,restrictor device keeping the spill flow through the said line substantially constant, and a control means in the said fuel supply line controlling the pressure at which the fuel is supplied to the said burner.

References Cited in the file of this patent UNITED STATES PATENTS 2,405,888 Holley Aug. 13, 1946 2,508,260 Holley May 16, 1950 2,643,513 Lee June 30, 1953 FOREIGN PATENTS 608,860 Great Britain Sept. 22, 1948 

